The present invention relates to a detecting method for detecting a physical quantity of an object under detecting on the basis of the nature of waves in a broad sense, which includes acoustic waves, ultrasonic waves and light, such as electromagnetic waves, elastic waves or the like. What are to be detected are physical quantities in a broad sense, which include physical characteristics of an object to be detected, such as a material to be detected. Specific examples of what are to be detected are:
A. Distance from a detecting device to an object to be detected, presence or absence of the detected object; PA0 B. Shape and position of the detected object; PA0 C. Various characteristics and a propagating speed of a medium when a wave propagates thereon or therethrough, the medium being located between a detecting device and an object to be detected:
An ultrasonic detecting method and an ultrasonic detecting device will be described by way of examples, for ease of explanation.
In the specification, a word "measurement" will frequently be used in addition to a word "detection", but the former is involved in the latter.
Of the ultrasonic detecting method and device, an ultrasonic measuring method and an ultrasonic measuring device, which use an ultrasonic wave, will first be described.
Generally, the ultrasonic detecting device means a device which projects a beam of an ultrasonic wave to an object, and receives and processes a reflected, scattered or refracted ultrasonic wave from the object, whereby detecting the object, measuring a distance from the detecting device to the object and the shape of the object, visualizing the object, or recognizes the object. The ultrasonic detecting device also means a device for measuring a sonic speed of an acoustic wave traveling in an object or an ultrasonic wave propagating medium. The object involves any object located in any medium which allows an ultrasonic wave to propagate, such as gas including air, liquid including water, sea water, or solid.
This type of a conventional ultrasonic measuring device will be described using an example in the field of ultrasonic nondestructive inspection. An ultrasonic measuring device functionally illustrated in FIG. 42 is described in "New Nondestructive Inspection Handbook" (referred to as an article A), edited by Nihon Nondestructive Inspection Association Corporation, Oct. 15, 1992, pp. 256 to 278, issued by Nikkan Kogyo Shinbun Company Ltd.
In FIG. 42 reference numeral 1 designates a pulser; 2, a receiver, 24, a timing signal generator portion; 4, a horizontal sweeper portion; 5, a display unit; 6, a probe; 7, a test piece; and 8, a defect. In the figure, the test piece 7 corresponds to an ultrasonic wave propagating medium, and the defect 8 corresponds to an object.
The operation of the ultrasonic measuring device thus constructed will be described. An impulse method is employed in the ultrasonic measuring device as shown in FIG. 42, which is used in the field of the ultrasonic nondestructive inspection. For the impulse method, reference is made to "Ultrasonic Defect Probing Method (revised edition)" (referred to as an article B), edited by the 19th Committee of Steel Making in Nihon Academy Promotion, Jul. 30, 1974, pp. 114 to 140, issued by Nikkan Kogyo Shinbun Company Ltd. The pulser 1 generates an electric pulse at the timing defined by a transmission repetitive frequency. The pulse width of the electric pulse is narrow to such an extent that it could be considered as an impulse, as shown in FIG. 43(a). The electric pulse is applied to the probe 6 where it is converted into an ultrasonic pulse as shown in FIG. 43(b). The ultrasonic pulse is projected to the test piece 7.
The ultrasonic pulse propagates through the test piece 7, and is reflected by the defect 8 in the test piece 7 and the bottom of the test piece 7, to thereby return to the probe 6. The probe 6 receives the returned ultrasonic pulse in the form of a reflection echo electric signal. The echo signal is amplified and rectified by the receiver 2. The output signal of the receiver 2 is transmitted to the display unit 5.
The timing signal generator portion 24 generates sync signals for controlling the operation timings of the respective circuits in the device. The horizontal sweeper portion 4 generates a time-axis (abscissa) sweep signal by using the sync signal for transmission to the display unit 5. Then, the display unit 5 displays a transmission pulse T, a reflection or defect echo F from the defect 8, and another reflection echo B from the bottom of the test piece 7, which are arrayed on the time axis, as shown in FIG. 42.
On the screen of the display unit 5, a position of the defect 8 in the test piece 7 is known by measuring a time position where the defect echo F appears. The size of the defect 8 is defined by the height of the defect echo F.
To improve the resolution in the measurement of distance to the defect 8 in the ultrasonic measuring device of this type, it is necessary to accurately measure the time position where the defect echo F appears. An extremely narrow time width of the defect echo F provides an accurate measurement of the time position. However, the actual ultrasonic pulse transmitted to the test piece 7 is such that even in the ultrasonic pulse of which the oscillation continuation time is short, the number of oscillation waves is approximately 1.5 to 3 as shown in FIG. 43(b).
Accordingly, the time width of the defect echo F is also such a length. An oscillating waveform of the ultrasonic pulse depends largely on the characteristic of the probe 6. In the probe 6 having a narrow frequency response characteristic, the number of the oscillation waves is large, while in the probe having a broad frequency response characteristic, it is small. In other words, the oscillation continuation time is inversely proportional to the band-width. Even in the probe 6, currently used as a broad band-width probe, the number of oscillation waves is 1.45 to 3 waves at most, as described on page 263 in the article A. Therefore, the resolution in the distance measurement is the half of the number of oscillation waves, i.e., 0.7 to 1.5 waves, even if the fact that the ultrasonic pulse travels to and from the defect 8 is taken into consideration.
The amplitude of an envelope of the defect echo gradually increases with time, and decreases after it reaches its peak. By making use of this amplitude variation, the resolution can be improved over the above-mentioned one when the lapse of time from an instant that the envelope rises till it reaches the peak is measured or a time point where the envelope rises from zero is measured. Actually, it is difficult to exactly measure these time points, however. The reason for this is that a variation of the amplitude is gentle in the vicinity of the time point where the amplitude is peaked and the time point where it rises.
To cope with this, a possible measure is present. In this measure, a threshold value is set at a position on the amplitude envelope, where is a preset value lower than the peak value. A time point where the amplitude varying along the envelope reaches the threshold value is measured. This measure also suffers from problems in that the accuracy and stability of the measurement are not satisfactory. The reason for this is that the echo waveform varies depending on the probe used and the shape of the object 8.
In some of the ultrasonic measuring devices and the ultrasonic microscopes in the field of the ultrasonic nondestructive inspection, the probe 6 is excited by using a burst signal as shown in FIG. 44, not the impulse as shown in FIG. 43(a). Where the burst signal is used, the oscillation continuation time of the reflection echo signal is longer than that in the case of FIG. 43(b). The result is an inaccurate measurement of the time positions where the reflection echoes appear.
In this type of the ultrasonic detecting device and method, for example, the ultrasonic measuring device, the measuring accuracy is unsatisfactory in measuring a time point where a reflection echo signal is received from an object. And the resolution in the distance measurement up to the object 8 is poor. The same thing is true for a case where a refractive echo rather than a scattering echo is received from an object 8, to thereby gain information of the object. This problem arises also in the measurement for the purpose of gaining information on an object 8 by receiving and processing an echo signal as well as the measurement for the purpose of measuring a distance to an object 8. The former measurement includes the measurements for grasping a shape of an object 8, visualizing the object, and discriminating the object. This leads to poor measuring accuracy in a case where an object 8, which allows an ultrasonic wave to propagate therein, is measured in its thickness and a sonic speed of the ultrasonic wave when it propagates therein.
For the above-mentioned background reasons, the present invention is to provide a detecting method which improves the resolution, or the detection accuracy, by using a signal of a multiple of frequencies, and a detecting device for executing the detecting method. For conventional detecting devices using an ultrasonic wave signal of a multiple of frequencies, reference is made to Published Unexamined Japanese Patent Application Nos. Hei. 4-24580, 4-286952, 2-136135, 5-123320, 5-200024, and 6-229991, for example.
The ultrasonic detecting device disclosed in the publication of Hei. 4-24580 is constructed on the basis of the fact that when an ultrasonic wave propagates in an ultrasonic wave propagating medium, the frequency characteristic on its attenuation depends on the propagating path of the ultrasonic wave. Two ultrasonic wave signals of high and low frequencies are used. To detect an object, one of the ultrasonic wave signals is selected which is suitable for the detection of the object. The ultrasonic detecting device disclosed in the publication of Hei. 4-286952 is constructed so as to detect a defect 8 on the basis of the fact that when an ultrasonic wave propagating medium (insulator) suffers from a defect located near to the end face thereof, an intensity of the end face echo of the ultrasonic wave varies depending on the frequency of the ultrasonic wave. The ultrasonic detecting devices disclosed in the publication of Hei. 2-136135, 5-123320, and 5-200024 are constructed on the basis of the fact that when an ultrasonic wave propagates in an ultrasonic wave propagating medium, its attenuation depends on the frequency thereof.
The detecting device disclosed in the publication of Hei. 6-229991 uses two burst signals (transmission signals) of different frequencies. In the detecting device, the phases of two detecting signals (echo signals) of those burst signals are detected, a difference between the two phases is obtained, a frequency is produced which is defined by the difference between the two phases and the difference between the two frequencies. The resultant frequency is used for measuring a distance from the detecting device to an object. The detecting method of the publication is equivalent to the detecting method using a single burst signal of a specific frequency, which is defined by the difference between the two frequencies. In this respect, this conventional art is different from the present invention in which two signals of different frequencies are provided, a candidate is selected from the two signals, and a distance to an object to be detected is determined using the candidate.
While those conventional art disclosed in the publications of Hei. 4-24580, 4-286952, 2-136135, 5-123320, 5-200024, and 6-229991 each use two signals of different frequencies, those are different from the present invention in the objects and the constructions as will be described hereinafter.